Stiffening and/or strengthening a structural member using a pre-made microtruss insert

ABSTRACT

A method for fabricating a micro-truss structure having particular application for a vehicle impact structure. The method includes providing a thin support enclosure having a plurality of walls, where at least one of the walls is transparent to ultraviolet (UV) light. The support enclosure is filled with a liquid photomonomer resin and light from at least one UV source is directed through apertures in a mask and through the at least one transparent wall into the enclosure so as to polymerize and at least partially cure columns of the resin therein to form polymerized struts defining a micro-truss core in the enclosure that is secured to the walls. The support enclosure is then inserted into an outer structural enclosure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for fabricating amicro-truss structure within an enclosure and, more particularly, to amethod for fabricating a micro-truss structure within a thin supportenclosure that includes shinning UV light into the enclosure through aUV transparent wall that forms at least part of the enclosure, and theninserting the support enclosure into an outer structural enclosure.

2. Discussion of the Related Art

Modern vehicles are equipped with a number of impact beams providingstructural integrity against collisions and impacts with objects, suchas other vehicles. More particularly, impact beams are traditionallyused in vehicle designs to protect occupants from front, side and/orrear impacts by absorbing energy through deformation in the event of avehicle crash and distributing the applied dynamic loads to other energyabsorbing sub-systems on the vehicle. For example, it is known toprovide impact beams in a front energy management or bumper assembly, arear energy management or bumper assembly and side impact assemblies ona vehicle. Impact beams at the front and rear of the vehicle are usuallyreferred to as bumper beams, and impact beams on the sides of thevehicle are sometimes referred to as anti-intrusion bars. In all cases,it is desirable to provide an impact beam with low mass, high flexuralstiffness and strength, and high energy absorption per unit mass. Thelightweight requirement is predicated by fuel economy standards and thefact that impact beams are located both very close to and very far fromthe vehicle's center of mass. Maximizing the flexural stiffness andstrength is necessary if the beam is to survive low speed impactswithout damage and transfer impact loads throughout the duration of ahigh speed impact event. Further, a high level of energy absorptiontranslates into reduced load transfer to the occupants of the vehicle,thus increasing safety.

In one known vehicle front energy management system, an impact beam iscomprised of a top and bottom facesheet in combination with an internalstructural core for providing high energy impact resistance in a lightweight and cost effective manner. Typically, the impact beam for such asystem includes aluminum, steel, carbon fiber, etc. layers that areextruded, roll-formed, etc. A hard energy absorbing layer may be formedon the impact beam having the general shape of an outer fascia trimpanel on the side closer to the fascia and that of the front face of theimpact beam on the side closer to the impact beam. A soft energyabsorbing layer is then formed on the hard energy absorbing layer andthe front fascia panel is then provided over the soft energy absorbinglayer. The combination of the hard energy absorbing layer and the softenergy absorbing layer provides a transition between the impact beam andthe front fascia panel so as to allow the system to conform to thedesired shape of the front fascia panel which may have significantangles and forms required by the vehicle styling. The hard energyabsorbing layer and the soft energy absorbing layer also provide atransition between the fascia panel and the impact beam to effectivelyabsorb low speed impacts without compromising system integrity.

It is known in the art to provide vehicle impact beams having sandwichstructures. These prior art impact beams can generally be categorizedinto three designs, namely, hollow beams that are fully or partiallyreinforced with a polymer or metallic foam, single or dual-sidedfacesheets reinforced with a honeycomb-like cellular core, and formedcomposite impact beams. For hollow metallic or polymer matrix compositetube structures which are fully or partially reinforced with alightweight foam core, the material used for the core can be either ametallic or polymeric foam that is bonded, mechanically attached orinterference fit into the tube structure. The purpose of the core is tocarry shear loads in the sandwich structures and absorb energy in theevent of a low or high speed impact, which is a distinction dependent onthe density and composition of the foam. The use of honeycomb orhoneycomb-like ordered cellular cores to provide reinforcement to one ortwo flat facesheets have an open-sided sandwich designs and havehoneycomb, discrete-stiffened or wine-crate structures extending fromthe front face of the impact beam back towards the passenger compartmentof the vehicle. If a second facesheet is not included between the coreand the passenger compartment, then the core material must be relativelydense since it provides most of the flexural stiffness to the structureadjacent to the shear load transfer.

For continuous or discontinuous fiber reinforced polymer matrixcomposite impact beams, the matrix material may either be athermoplastic or thermosetting polymer introduced via resin transfermolding, compression molding, blow molding, or other similar fabricationprocesses.

It is also known in the art to fabricate a three-dimensional network ofphotopolymer waveguides comprising a unitary truss or latticearchitecture, hereafter referred to equivalently as micro-truss ormicro-lattice. For example, U.S. Pat. Nos. 7,653,279 and 7,382,959disclose a process for fabricating such a micro-truss structure.Generally, the process includes providing a reservoir or mold filledwith a volume of a curable monomer and covered by a mask includingstrategically positioned apertures. UV light sources are positionedrelative to the mask and exposure to collimated UV light through themask apertures forms a series of interconnected self-propagatingphotopolymer waveguides, referred to herein as struts, to form the trussor lattice architecture. Once the photopolymer waveguides are formed,the reservoir is emptied of the unpolymerized monomer which was notexposed to UV light. The micro-truss structure may then undergo apost-cure operation to increase the cross-link density in thephotopolymer waveguides. This post-cure may be accomplished via athermal cure, an additional exposure to UV light, an equivalent methodor combination thereof.

SUMMARY OF THE INVENTION

The present disclosure describes a method for fabricating a micro-trussstructure having particular application for a vehicle impact structure.The method includes providing a thin support enclosure having aplurality of walls, where at least one of the walls is transparent toultraviolet (UV) light. The support enclosure is filled with a liquidphotomonomer resin and light from at least one UV source is directedthrough apertures in a mask and through the at least one transparentwall into the enclosure so as to polymerize and at least partially curecolumns of the resin therein to form polymerized struts defining amicro-truss core in the enclosure that is secured to the walls. Thesupport enclosure is then inserted into an outer structural enclosure.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a vehicle showing locations on the vehicle whereimpact beams may be provided;

FIG. 2 is a cross-sectional type view showing a known front bumperenergy management system for a vehicle;

FIG. 3 is an isometric view of a curved impact beam including a top andbottom facesheet with a micro-truss structure formed therebetween;

FIG. 4 is an isometric view of a micro-truss architecture suitable forthe various micro-truss structures described herein;

FIG. 5 is a cross-sectional type view showing a front bumper energymanagement system for a vehicle including two micro-truss layers;

FIG. 6 is an exploded side view of an energy management system includingtwo micro-truss layers;

FIG. 7 is an isometric view of a micro-truss structure and enclosure;

FIG. 8 is an exploded isometric view of the structure shown in FIG. 6being slid into a supporting structure; and

FIG. 9 is an isometric view of the micro-truss structure shown in FIG. 8and being cut at segments to make it flexible.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa method for fabricating a micro-truss structure within a supportenclosure is merely exemplary in nature, and is in no way intended tolimit the invention or its applications or uses. For example, the methodhas particular application for fabricating impact structures forvehicles. However, as will be appreciated by those skilled in the art,method may have application for other structures.

As will be discussed in detail below, the present invention proposes anumber of structures and related methods for providing an automotiveimpact beam assembly with a sandwich construction incorporating anarchitected core material, referred to herein as a micro-truss structureor micro-truss core. The architected core material is composed of arepeating set of ordered three-dimensional polymer waveguides. Althoughsandwich structures are known in the vehicle impact beam art, whatdifferentiates the present invention from other designs is the use ofand formation process for the micro-truss sandwich core. As will bediscussed, a sandwich structure with an architected micro-truss coreenables impact beam capabilities and performances that were heretoforenot possible using known methods.

The micro-truss core material possesses increased stiffness and strengthversus alternative stochastic sandwich core materials, such as metal orpolymeric foams, given that it enables precise control of the material'sspatial arrangement. Higher stiffness and strength in the sandwich coretranslates into a reduced repair cost in the event of low speed impacts.Micro-truss core materials can be manufactured at much lower costs ascompared to other high-strength and high-stiffness core materials, suchas metallic or composite honeycombs. Because the micro-truss core isformed in place and directly bonded onto curved surfaces, fasteners andangled faces, it eliminates the need for costly secondary machining andmulti-stage bonding operations associated with honeycomb cores. Themicro-truss fabrication process enables functional grading, spatialcontrol of the physical and mechanical properties of the core in allthree-dimensions using a single material, something which is notpossible with all of the other sandwich core forms that have homogeneousphysical and mechanical properties at different locations in the core.In an impact beam, functional grading can be used to provide enhancedstructural support in localized regions of the beam, such as attachmentpoints, the beam mid-span or likely impact locations. Polymermicro-truss core materials can be integrated into sandwich designscontaining aluminum, steel, or carbon fiber reinforced polymerfacesheets, or any combination thereof, without the need for corrosionprevention devices or coatings. This is not the case for core materialssuch as aluminum honeycombs or foams which require galvanic protectionwhen applied to steels or carbon fiber reinforcement polymers. Thesurfaces of front and rear impact beams are often covered with a softenergy absorbing layer to prevent injury to pedestrians under low speedimpact conditions. This energy absorbing material is added afterfabrication of the beam, thus increasing part count and manufacturingcomplexity. In comparison, the same micro-truss process can be used toform both the soft energy absorption layer and the hard energyabsorption layer by changing the thickness, orientation, density andchemical composition of the photopolymer waveguides in each layer. Theformation of an integral energy absorbing layer on the outside of theimpact beam during manufacture of the sandwich construction saves time,cost and reduces the associated part count.

The use of a sandwich construction, particularly one with a micro-trusscore, also opens up the design space for impact beams to form geometriesthat cannot be manufactured using monolithic metal for fiber reinforcedpolymer composites alone. The sandwich design can circumvent inherentlimits for extrusion, pultrusion, stamping, and laminating processes byutilizing simple shapes for the metal or composite facesheet materialsto create a complex geometry sandwich.

Within the sandwich construction, the structure of the micro-truss coreresults in a higher stiffness and strength compared to prior artstochastic core materials. Additionally, the micro-truss architectureallows for the properties of the beam to be spatially tailored to meetanticipated impact conditions. The ability to functionally grade theperformance of the micro-truss core throughout the beam ensures thehighest level of structural efficiency, i.e., material is only placedwhere it is required. The method of fabrication presented herein alsooffers significant improvements over previous sandwich impact beammanufacturing methods. The net shape formation of the micro-trussresults in a lower-cost structure as compared to an equivalentperformance honeycomb core as a result of the labor-intensive machiningand multi-stage bonding operations required for incorporation of thehoneycomb structure. Furthermore, the same micro-truss fabricationprocess can be used to integrate additional parts, such as front andrear energy absorbing padding, into the structure of the impact beamcutting down on part counts and manufacturing time.

Along the length direction, the impact beam may have a non-prismaticcross-section, where the cross-section may vary along the length eitherby changing the thickness of the facesheets, the thickness of the core,or the density, architecture or composition of the core, i.e.,functional grading. In certain embodiments of the invention, the beamhas curvature along one axis such that the surface normal vectors of thesandwich cross-section are not parallel to one another along the lengthof the beam. In an additional embodiment, the beam may be straight sothat all beam cross-section surface normals are parallel along thelength dimension of the beam. In one embodiment, the width dimension ofthe impact beam is sealed against air or fluid flow such that the twofacesheets comprising the outer surface of the beam form a closedperimeter. Alternately, the cross-section of the beam can be open suchthat the top and bottom facesheets do not connect and are attached onlyby the architected core.

In addition to the architected micro-truss material forming the core ofthe sandwich impact beam, other micro-truss materials may beincorporated at the top facesheet surface of the beam to function as anenergy absorption padding material. These energy absorbing truss layersare supported only on one side by the facesheet, with the outer-facingsurface unconstrained or bonded to a thin facesheet. Typically, theseenergy absorbing truss materials are composed of different polymerchemistries compared to that used for the impact beam sandwich core suchthat the outer energy absorbing material is more compliant than theinner structural reinforcing core.

In the design of the impact beam, mechanical attachment or supporthardware features may be added to the beam to couple it to thesurrounding vehicle structure or provide functionality in the event of acrash. Particularly, the micro-truss core may be formed directly aroundinternally or externally threaded inserts that provide locations formechanical attachments passing through one or both facesheet surfaces.Tow lug devices may also be added to the sandwich impact beam structureto provide a hard point for tow line attachment in the event that thevehicle is disabled. The tow lug devices may be fastened to the impactbeam, using the aforementioned fastener integration process, bondeddirectly to the facesheet or welded directly to the beam if one of thesandwich facesheets is metallic.

Continuous carbon fiber reinforced polymer matrix composites can be usedto form the facesheets of the sandwich impact beam. However, themicro-truss fabrication process described herein is compatible with avariety of different materials including discontinuous carbon fibercomposites, continuous or discontinuous glass fiber composites,unreinforced polymers, aluminum alloys, structural steels, or anycombination thereof. This means that both facesheets in the impact beammay be composed of the same material or they may be composed ofdissimilar materials. The latter case is highly beneficial when thecritical property determining material selection (e.g. stiffness, mass,cost, formability, etc.) is different for both facesheets. However, ifdissimilar materials are used for the facesheets, then the opencross-section design may be required to prevent galvanic corrosion orsimilar material capability issues between the facesheets themselves andnot the core.

The micro-truss core structure in the sandwich impact beam is composedof an ordered three-dimensional network of self-propagating polymerwaveguides grown from a photo-monomer resin using a controlled exposureto collimated UV light sources at specified orientations through aplurality of apertures in a mask. Any UV-curable photo-monomer or blendthereof that displays the self-propagation phenomenon can be used toform the micro-truss or micro-lattice architecture. Alternatively, themicro-truss may be formed using the aforementioned photo-polymerizationtechnique and then converted or augmented via electro or electro-lessplating, reverse casting, vapor deposition, oxidation, dip coating,sputtering, or other suitable process into a hollow or solid metallic,ceramic, or polymer material different from the initial photopolymer. Incertain embodiments of the impact beam manufacturing process, themicro-truss core is grown directly on one of the two sandwichfacesheets. The second facesheet is then joined to the core using anadhesive material applied to the facesheet and/or the micro-truss. Thisadhesive may be composed of a single or multi-part paste for acontinuous sheet of film. Additional materials, such as glass, scrim orsyntactic fillers, may be added to the adhesive to maintain control ofthe bond line thickness.

Fabrication of the top and bottom facesheets is the first step forproduction of the impact beam being outlined herein. In one embodiment,these facesheets are composed of a continuous carbon fiber reinforcedthermosetting or polymer composite produced from pultruded sections. Inaddition to these embodiments, the facesheets may be produced withdiscontinuous carbon fiber reinforcement, continuous or discontinuousglass fiber reinforcement, thermoplastic polymer matrices, unreinforcedpolymers, metallic facesheets, such as steels, aluminum alloys, etc., orany combination thereof. Consequently, manufacturing methods suited toeach type of material may be used to produce the facesheets, including,but not limited to, thermoforming, spraying, injection molding, resintransfer molding, blow molding, stamping, casting, brake forming, rollforming, stretch forming, drawing, etc.

The top and bottom facesheets are formed in order to obtain an open orclosed sandwich profile. Additionally, each facesheet may have aprismatic or non-prismatic cross-section along the beam length dependingon the beam design and capabilities of the manufacturing processes usedto form the beam, i.e., pultruded facesheets must be prismatic. Adhesionpromoters or conversion coatings may also be applied to the surfaces ofthe facesheets in contact with the micro-truss core in order to form ahigh strength adhesive bond.

When the impact beam design contains curvature along one or more axisone method of introducing curvature is to start with flat facesheetmaterials and form them into the final curved shape after formation ofthe micro-truss core. In this case, both top and bottom facesheets donot contain any initial curvature prior to the growth of the core. Inadditional embodiments, the facesheet materials posses an initialcurvature, i.e., are not formed flat, and are fixtured into a flat shapeduring the micro-truss fabrication process, and then allowed to resumetheir curved shape upon release from the fixture.

The next step in the impact beam fabrication method is to form themicro-truss core shape directly onto one or both of the facesheets. Themicro-truss formation process consists of placing a photo-monomersolution in a mold in contact with one of the facesheets, masking off atwo-dimensional area on the opposite surface of the facesheet and mold,exposing the liquid monomer to collimated UV light sources at specificorientations through the patterned mask, and then removing the mask,mold and excess monomer to produce a networked three-dimensional polymerstructure bonded to the surface of one or both of the facesheets. If onefacesheet in the sandwich structure is transparent at the wavelength ofcollimated light used to form the micro-truss, then the entire sandwichstructure may be formed in a single operation by exposing the monomer tothe UV light sources through the transparent facesheet. Subsequently, inthis embodiment, the micro-truss forms and bonds onto both facesheetsurfaces in contact with the monomer. If both of the facesheets are nottransparent at the wavelength of collimated light used to form themicro-truss, then one facesheet must be affixed to the exposedmicro-truss surface using a secondary process as described in thefollowing sections.

If the facesheets are initially curved, then one of the facesheets mustbe fixtured in a flat configuration prior to formation of themicro-truss. Conversely, if the facesheets are initially flat, then noadditional fixturing is required. If the sandwich impact beam contains aclosed section design, then the mold containing the liquid monomer resinis not required on the closed side of beam, but only at the end pointsof the beam if these sections are open. During the micro-truss formationprocess, internally or externally threaded fastener inserts may beinserted into the liquid monomer to form bonded-in-place mechanicalattachment points in the core.

Regardless of the curvature, transparency, and section properties of thebeam, following formation of the core on one of the facesheets, theexcess monomer resin, mold, and pattern mask are removed. The mold andthe mask are then cleaned for re-use and the un-polymerized resin may berecycled and used to form the core of a subsequent impact beam. Thethree-dimensional networked polymer micro-truss structure is thenpost-cured using a short duration exposure to high intensity UV light inorder to fully polymerize the structure and fix it in a finalconfiguration. Alternatively, the post-cure can be accomplishedthermally, i.e., in an oven. Prior to this post-cure process, themicro-truss can still undergo deformation without introducing a residualstress state in the core. Therefore, if the impact beam is designed withcurvature about one or more axis and the facesheet material is producedin a flat form, a fixture may be used to impart the required degree ofcurvature to the beam prior to post-curing of the core. Uponpost-curing, the beam is removed from the fixture with the impartedcurvature maintained by the stiff, post-cured micro-truss core.Conversely, if the facesheet is initially formed in a curved shape, andthen the post-cure is performed free standing.

Once the micro-truss core is post-cured in its final shape and if bothfacesheets are not transparent, the sandwich impact beam is assembled bybonding the top facesheet to the micro-truss core and also the bottomfacesheet if a closed section design is used. Whereas, the adhesive bondbetween the micro-truss core and the bottom facesheet is formed directlyduring a micro-truss formation process, the bond between the topfacesheet and the micro-truss requires a secondary operation. In oneembodiment, this secondary operation involves dip or roll coating alayer of paste adhesive onto the exposed top surface of the micro-trussand the bottom facesheet, and then placing the top facesheet in contactwith this adhesive layer. The bond surface of the top facesheet may havea conversion coating, primer or other adhesion promoter applied prior tothis assembly step. In additional embodiments, other bonding or joiningoperations may be used to affix the top facesheet to the micro-trusscore and the bottom facesheet including, but not limited to, fusionwelding, ultrasonic welding, adhesive film bonding, friction welding,spray adhesive bonding, interference fitting, mechanical attachment,etc. It is important to note that the same joining method does not needto be applied to the micro-truss/top facesheet interface as that appliedto the top facesheet/bottom facesheet interface.

In an additional embodiment, one or more micro-truss structures areadded to the top facesheet of the sandwich structure prior to bonding ofthe top facesheet to the impact beam. These additional micro-trussstructures function as hard or soft energy absorption layers, facingoutward from the vehicle to protect pedestrians who may come in contactwith the impact beam during a dynamic event. These energy absorbinglayers are added to the top facesheet in the exact same manner as thestructural micro-truss core is added to the bottom facesheet.

Once the top facesheet is applied to the impact beam, finishingoperations are applied to integrate the potential features describedabove. For instance, subtractive machining operations may be performedto trim sections of the beam or to provide holes for moisture drainagefrom the core. Additionally, support brackets or mechanical attachmentsmay be added to allow the beam to interface with and transfer loads tosurrounding vehicle components. If a tow lug is included in the beamdesign, it is also attached at this stage by mechanical fasteners or bybonding or welding to a facesheet.

FIG. 1 is a top view of a vehicle 10 including a front bumper impactbeam 12, a rear bumper impact beam 14 and side impact anti-intrusionbeams 16. The illustration of the vehicle 10 shown in FIG. 1 is intendedto give context to the invention discussed below to show locations onthe vehicle 10 where high impact resistant, but light weight structuralbeams are usually provided on a vehicle.

FIG. 2 is a cross-sectional type view of a known front energy bumper andmanagement system 20 of the type briefly discussed above. The energymanagement system 20 includes a front fascia panel 22 having a designand configuration for a particular vehicle style and is the show facefor the front bumper of the vehicle. Vehicle fascia's can come invarious shapes, sizes and materials, and are typically very thin andlight weight. For example, the fascia panel 22 can be a thinthermoplastic material configured to have a visually appealing showsurface, which can be formed by any suitable plastic molding process.The fascia panel 22 can have any suitable shape including hard angles,such as represented by crease 24.

The energy management system 20 includes an impact beam 26 at the backof the system 20, which provides the main structural integrity of thesystem 20 against impacts so as to better protect vehicle occupants. Theimpact beam 26 can be any suitable impact beam known in the art, such asimpact beams that are extruded or roll formed aluminum or steel. A hardenergy absorbing layer 28, such as a high density structural foam, issecured to the impact beam 26, such as by mechanical interference, glueor the like, and provides a transition between the impact beam 26typically having a slightly curved configuration readily able to beproduced by its forming process to the more highly curved fascia panel22. The hard energy absorbing layer 28 also provides energy absorptionduring impacts, generally low energy or slow speed impacts. A softenergy absorbing layer 30 is adhered to the hard energy absorbing layer28, such as by mechanical interference, glue or the like, and can be asofter foam layer that supports the fascia panel 22, provides a cushionthat mitigates injury during interaction with pedestrians and provides alayer having “give” against minor bumps. The fascia panel 22 restsagainst the soft energy absorbing layer 30.

The present invention proposes employing micro-truss sandwich structuresand beam assemblies for structural and/or energy management systems on avehicle, such as the front energy management system 20. As will bediscussed, the proposed energy management system employing themicro-truss structure will eliminate the need for three separatecomponents of the impact beam 26, the hard energy absorbing layer 28 andsoft energy absorbing layer 30 so as to reduce cost and weightcompromising performance.

FIG. 3 is an isometric view of a micro-truss core sandwich beam 40including a top facesheet 42, such as a layer of aluminum or carbonfiber composite, and a bottom facesheet 44 also being of a aluminum orcarbon fiber composite layer, where the facesheets 42 and 44 do not needto be made of the same material, and where the beam 40 has a generalcurved shape. A micro-truss structure 46 including polymerized struts 48is formed on the bottom facesheet 44 by the micro-truss fabricationprocesses known to those skilled in the art. The top facesheet 42 isthen secured to the strut nodes that define joined ends of the struts 48provided by the micro-truss fabrication process by gluing orthermo-bonding the sheet 42 to the nodes. In this embodiment, the beam40 is closed at the edges by edge panels 32, which may be a folded partof either of the sheets 42 or 44 or a separate panel welded or bonded tothe sheets 42 and 44, or secured thereto by some other means.

FIG. 4 is an isometric view of a micro-truss structure 34 includingpolymerized struts 36 of the type being discussed herein formed by knownmicro-truss fabrication techniques, and being suitable for themicro-truss structure 46, as well as various other micro-trussstructures being discussed herein. Nodes 38 are depicted at a topsurface of the structure 34.

The beam 40 in FIG. 3 is intended to be a general representation of thetype of curved beams that can be provided in a vehicle impact beamassembly as described herein. The micro-truss fabrication process can beconfigured and provided so that the micro-truss structure 46 can beformed on highly and intricately curved support sheets or panels. FIG. 5is a cross-sectional type view of a front energy management system 50having a front fascia panel 52 with the same configuration as the frontfascia panel 22 from FIG. 2. In this embodiment, the impact beam 26, thehard energy absorbing layer 28 and the soft energy absorbing layer 30from FIG. 2 are replaced with micro-truss core sandwich structures asdescribed. The functions of the hard energy absorption layer 30 and theimpact beam 26 from FIG. 2 are performed by a non-prismatic sandwichbeam with a micro-truss core 56, a top facesheet 64 and a bottomfacesheet 58. The function of the soft energy absorption layer isperformed by the lower effective modulus micro-truss layer 60 that isgrown on the top facesheet 64 of the sandwich beam. The soft energyabsorbing micro-truss structure 60 has a lower effective modulus thanthe micro-truss core 54, which can be provided by one or more of thethickness of the micro-truss structure, the type of material of thestruts 56 and 62, the size and diameter of the struts 56 and 62, thespacing of the struts 56 and 62, combinations thereof, etc. The sheet 64is adhered to the nodes of the micro-truss structure 54 in the samemanner as the facesheet 42 was adhered to the micro-truss structure 46.The front fascia panel 52 is then mounted such that it is in contactwith the top nodes of the micro-truss structure 60.

FIG. 6 is an illustration of an exploded micro-truss assembly 70 to helpshow the formation of the hard energy absorbing micro-truss structure 54and the soft energy absorbing micro-truss structure 60 referred toabove. The assembly 70 includes a hard energy absorbing micro-trussstructure 72 that includes a micro-truss 74 having polymerized struts 76formed on a facesheet 78. During manufacture of the structure 72, thearea where the micro-truss 74 will be formed is blocked off by asuitable enclosure (not shown) and a mask is provided over theenclosure. Resin is then poured into the enclosure and multiple lightsources expose the resin to collimated light through the apertures ofthe mask to form the struts 76. While the struts 76 are in theirpartially cured state, the residual monomer resin is removed from theenclosure and the remaining partially cured micro-truss 74 is fullycured by a suitable heat source or UV radiation. The assembly 70 alsoincludes a soft energy absorbing micro-truss structure 80 including amicro-truss 82 having struts 84 formed on a top facesheet 86. Thestructure 80 is formed by the same process as the hard energy absorbingstructure 72 and when both of the structures 72 and 80 are fully cured,the top facesheet 86 is bonded to the micro-truss 74 by an adhesivebonding operation or other suitable joining process.

In the embodiment discussed above, the hard micro-truss energy absorbinglayer is formed separately from the soft micro-truss energy absorbinglayer and then the bottom sheet of the soft energy absorbing layer issecured to the top surface of the micro-truss structure of the hardenergy absorbing layer. The present invention also proposes reducingsome of these manufacturing steps in order to reduce costs by formingthe hard energy absorbing micro-truss structure with both a top sheetand a bottom sheet.

FIG. 7 is an isometric view of a micro-truss structure 90 including asupport enclosure 92. The micro-truss structure 90 is shown having asquare shape solely for discussion and illustrative purposes, where theparticular shape of the structure 90 can be any shape, width, height,length, etc. for a particular vehicle structural and/or energymanagement system or otherwise including the shapes shown in FIG. 5. Theenclosure 92 includes in this example six sidewalls 94 to define thecomplete support enclosure 92, where one of the sidewalls 94 is removedto show a micro-truss core 96 formed within the enclosure 92 includingpolymerized struts 98. The sidewalls 94 are comprised of a very thin andlow structural integrity material, such as thin plastic sheets, designedmerely to hold the liquid polymer resin within the enclosure 92 duringformation of the micro-truss core 96. Particularly, the enclosure 92 ismade as lightweight and flimsy as possible to perform its purpose ofholding the uncured resin so as to be able to conserve material andcost.

At least a top panel 100 of the support enclosure 92 is made of a UVtransparent material, such as acrylic, that allows the UV curing lightfor the polymer resin within the enclosure 92 to form the struts 98 inthe known manner. Particularly, a suitable mask 104 including apertures106 is positioned above the panel 100 and a configuration of UV lightsources 108 shine UV light through the mask 104 and through the panel100 so that it partially cures the polymeric resin within the supportenclosure 92 in the known manner to form the micro-truss core 96.Because the light hits the inside surface of all of the panels 94, themicro-truss core 96 is secured to all of the panels 94 of the enclosure92 when formed, and particularly the top and bottom panels of theenclosure 92. The panels 94 other than the top panel 100 can also bemade of the UV transparent material, or can be made of another lessexpensive material, if appropriate. Drain holes 102 are provided in theenclosure 92 to remove the un-cured liquid resin left therein after themicro-truss core 96 has been formed at least to a partially cured state.

As mentioned, the panels 94 of the enclosure 92 are very thin and do notprovide the structural integrity necessary for impact beams, energymanagement systems, structural elements, etc. for a vehicle. While themicro-truss core 96 is in a partially cured state within the supportenclosure 92 it is inserted into a structural enclosure that providesthat function. FIG. 8 is an exploded isometric view of a structuralassembly 110 including the micro-truss structure 90 and an outerstructural enclosure 112 in which the structure 90 is inserted.Particularly, the structural enclosure 112 includes four side panels 116defining an open inside area 114 having an open side 118 through whichthe structure 90 is inserted. Although the structural enclosure 112 isshown having the same shape as the structure 90, the structuralenclosure 112 can have a slightly different shape than the structure 90.Because the micro-truss core 96 is only partially cured and theenclosure 92 is very thin, the structure 90 is pliable and flexible sothat it can be bent and readily conformed to the shape of the outerenclosure 112. Once the structure 90 is inserted into the enclosure 112it is secured thereto in any suitable manner, such as by bolts 120, anadhesive or other suitable fasteners. The micro-truss core 96 is alsofully cured at this time, such as by placing the assembly 110 in anoven. The combination of the outer enclosure 112 and the enclosedmicro-truss structure 90 forms a structural member that can be designedfor stiffness, strength, energy absorption, etc. The enclosure 112 canbe made of any suitable material for a particular application, such as ahigh modulus strength wear resistant, chemical resistant material, andcan be made by any suitable technique, such as by roll forming. Further,the structure 112 can have the particular shape of the energy managementsystem or other application desired. By separating the fabricationprocess for the structure 90 and the outer enclosure 112, differentprocesses and fabrication techniques can be employed for efficiencyvolume of the produced parts.

Although the outer enclosure 112 is shown as a single piece member, inother embodiments, the enclosure 112 can be configured as multiplepieces, where, for example, the enclosure 92 is inserted into one pieceand another piece is then placed over the first piece and securedthereto. It is further noted that the micro-truss core 96 need notspecifically match the shape of the outer enclosure 112, where thestructure 90 only fills some of the space of the enclosure 92.

The discussion above includes curing the micro-truss core 96 to itsfinal rigid state after the structure 90 has been inserted into theouter enclosure 112 so as to allow it to be readily shaped to the outerenclosure 112 before it is fully cured. However, for certain fabricationprocesses, it may be desirable to fully cure the micro-truss core 96before the structure 90 is inserted into the enclosure 112. According toanother embodiment of the invention, the micro-truss core 96 is fullycured in the enclosure 92 using normal micro-truss structure fabricationtechniques. If the resin is of a proper thermoset material and theenclosure 92 is of a suitable type of material, heating the micro-trusscore 96 and the enclosure 92 above a known transition temperature T_(g)for the particular materials causes those materials to become pliableand flexible, where they can then be conformed to the enclosure 112while being held above the transition temperature. The temperature T_(g)is below a temperature where the structure 90 will become irreversiblydamaged. Once the structure 90 is in the enclosure 112 to form theassembly 110, then the temperature can be reduced below the transitiontemperature T_(g) where the micro-truss core 96 will return to its rigidcured state. This process of heating the micro-truss core 96 after ithas been fully cured to allow it to be conformed to the desired shapemay have advantages above those of the post curing micro-truss structureprocesses referred to above because it will allow a repeated shapechange to better fit within the processing needs of the sandwichmicro-truss structure beam.

FIG. 9 is an isometric view of the structure 90, which shows thestructure 90 having more length than in FIG. 6, but being of the sameconfiguration. If the outer structural enclosure 112 has a curvedconfiguration, then it may be desirable to form slots 122 in the panels94 at certain intervals along the length of the structure 90 to formflexible segments 124 in the structure 90 that allow it to more easilyconform to the curved shape of the outer enclosure 112.

The foregoing discussion disclosed and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A method for fabricating a micro-truss structure,said method comprising: providing a thin support enclosure including aplurality of walls where at least one of the walls is transparent toultraviolet (UV) light; filling the support enclosure with a liquidphotomonomer resin; directing light from at least one UV source thoughapertures in a mask and through the at least one transparent wall intothe support enclosure so as to polymerize and at least partially curethe resin therein to form polymerized struts defining a micro-truss corein the support enclosure secured to the walls; providing an outerstructural enclosure; and inserting the support enclosure into thestructural enclosure and securing it thereto.
 2. The method according toclaim 1 further comprising draining uncured liquid resin from thesupport enclosure after the micro-truss core has been at least partiallycured.
 3. The method according to claim 2 further comprising fullycuring the micro-truss core after the support enclosure has beendrained.
 4. The method according to claim 3 wherein inserting thesupport enclosure into the structural enclosure includes inserting thesupport enclosure into the structural enclosure while the micro-trusscore is in a partially cured state.
 5. The method according to claim 4further comprising fully curing the micro-truss core after the supportenclosure is inserted into the structural enclosure.
 6. The methodaccording to claim 3 wherein fully curing the micro-trust core includesfully curing the micro-truss core before the support enclosure isinserted into the structural enclosure.
 7. The method according to claim6 further comprising heating the support enclosure and the fully curedmicro-truss core to a temperature that the support enclosure and themicro-truss core become bendable and pliable, but are not damaged, theninserting the support enclosure into the structural enclosure, and thenallowing the support enclosure and the micro-truss core to cool andbecome rigid.
 8. The method according to claim 1 wherein the supportenclosure and the structural enclosure have different shapes.
 9. Themethod according to claim 1 wherein the structural enclosure is a singlepiece enclosure.
 10. The method according to claim 1 wherein thestructural enclosure is a multiple piece enclosure where the supportenclosure is positioned in one piece of the structural enclosure andthen another piece of the structural enclosure is positioned over thesupport enclosure.
 11. The method according to claim 1 wherein providinga support enclosure including a plurality of walls includes providing asupport enclosure including only one wall transparent to UV light. 12.The method according to claim 1 wherein providing a support enclosureincluding a plurality of walls includes providing a support enclosureincluding all of its walls being transparent to UV light.
 13. The methodaccording to claim 1 wherein providing a support enclosure including aplurality of walls includes providing a support enclosure including sixwalls.
 14. The method according to claim 1 further comprising cuttingsegmented slots in the support enclosure to make the support enclosuremore bendable before the support enclosure is inserted into thestructural enclosure.
 15. The method according to claim 1 wherein themicro-truss structure is a vehicle structure.
 16. The method accordingto claim 15 wherein the vehicle structure is an impact beam.
 17. Amethod for fabricating a micro-truss structure for a vehicle, saidmethod comprising: providing a thin support enclosure including aplurality of walls where at least one of the walls is transparent toultraviolet (UV) light; filling the support enclosure with a liquidphotomonomer resin; directing light from at least one UV source thoughapertures in a mask and through the at least one transparent wall intothe support enclosure so as to polymerize and partially cure the resintherein to form polymerized struts defining a micro-truss core in thesupport enclosure secured to the walls; draining uncured liquid resinfrom the support enclosure after the micro-truss core has been partiallycured; providing an outer structural enclosure; inserting the supportenclosure into the structural enclosure and securing it thereto; andfully curing the micro-truss core after the support enclosure isinserted into the structural enclosure.
 18. The method according toclaim 17 wherein providing an enclosure including a plurality of wallsincludes providing an enclosure including only one wall transparent toUV light.
 19. The method according to claim 17 further comprisingcutting segmented slots in the enclosure before the enclosure isinserted into the outer enclosure to make the support enclosure morebendable.
 20. A method for fabricating a micro-truss structure for avehicle, said method comprising: providing a thin support enclosureincluding a plurality of walls where at least one of the walls istransparent to ultraviolet (UV) light; filling the support enclosurewith a liquid photomonomer resin; directing light from at least one UVsource though apertures in a mask and through the at least onetransparent wall into the support enclosure so as to polymerize andpartially cure the resin therein to form polymerized struts defining amicro-truss core in the support enclosure secured to the walls; draininguncured liquid resin from the support enclosure after the micro-trusscore has been partially cured; fully curing the micro-trust core;heating the support enclosure and the micro-truss core to a temperaturethat the support enclosure and the micro-truss core become bendable andpliable, but are not damaged; providing an outer structural enclosure;inserting the heated support enclosure into the structural enclosure andsecuring it thereto; and allowing the support enclosure and themicro-truss core to cool and become rigid.
 21. The method according toclaim 20 further comprising cutting segmented slots in the enclosurebefore the enclosure is inserted into the outer enclosure to make thesupport enclosure more bendable.